In order to enhance the developing properties and the like of a toner, the surface of toner particles is modified by heat treatment. PTL 1 describes a method for spraying and dispersing powder particles into hot air with compressed air, thereby modifying the surface, and spheroidizing the particles. PTL 2 describes a method for adding an additive such as silica to the powder particles, then subjecting the powder particles to heat treatment to fix the additive onto the powder particles, and thereby removing the liberated additive.
However, in the method of modifying the surface by using heat, when more heat than is necessary is applied to the toner, the toners fuse with each other and occasionally produce coarse particles. In order to efficiently and stably manufacture surface-modified particles for the toner, which do not thus contain the coarse particle, contain less fine powder, and have a sharp particle size distribution, there is a room for improvement in points of an apparatus and method for manufacturing the toner.
In addition, in order that the toner acquires adequate developing properties and the like, the toner can have an average degree of circularity of 0.960 or more.
However, when an average degree of circularity of the toner is high, excessively spheroidized toner particles generally increase in the toner, and thereby, cleanability tends to be lowered. This is because the excessively spheroidized toner particles easily pass through a cleaning blade. Techniques for preventing the toner particles from passing through the cleaning blade include increasing the contact pressure of the cleaning blade, but the technique has a limit because of causing a harmful effect such as the increase of the running torque of a drum and the wear of the cleaning blade. Incidentally, it is proved from a recent investigation that when the frequency of the particles with the degree of circularity of 0.990 or more in the distribution of the degree of circularity of the toner exceeds 20%, a cleaning failure tends to occur easily.
PTL 1 proposes a method of subjecting a so-called pulverized toner to heat treatment, and thereby adequately spheroidizing the toner. However, when the toner particles are heat-treated by using the technique described in PTL 1, the toner particles are nonuniformly heat-treated, and accordingly the heat-treated toner particles and untreated toner particles result in being mixed. On the other hand, when all of the particles are heat-treated for a long period of time so as to be uniformly heat-treated, the ratio of the excessively spheroidized toner particles increases in the toner, and the cleanability of the toner is lowered.
PTL 3 describes a method of heat-treating toner particles while making the air flow in the apparatus uniform to some extent, as a technique of uniformly heat-treating the toner particles.
However, in the method described in PTL 3, the portion of charging the toner is provided in the charge portion of hot air, and accordingly the toner is instantly heat-treated in a narrow range. In this case, the toner is not sufficiently dispersed, and the increase of the coarse particles due to the fusion of the toners is concerned. In addition, when the treatment amount is increased, the heat treatment efficiency of the toner is rapidly lowered, because the toner concentration with respect to the hot air increases. Accordingly, the heat-treated toner and the untreated toner result in being mixed.
PTL 4 describes a structure in which the charge position of the hot air and the charge position of the toner are reversed, in contrast to the structure described in PTL 3. However, in the structure described in PTL 4, the toner is injected toward the hot air and also toward the center part of the apparatus, and accordingly it is concerned that the heat-treated toners collide with each other before being solidified and the coalescing particles increase.